I mean RPC, because that is the common use case of IPC between applications and servers in microkernel systems. It is more expensive than shared library calls o

I mean RPC, because that is the common use case of IPC between applications and servers in microkernel systems. It is more expensive than shared library calls or system calls no matter how much you optimize it on current architectures. I'm also well aware of L4's performance vs Mach, but like I said: "instead of the lingering 'eh, it's a little slower but we can ignore that,' exokernels provide much better opportunities for optimization and tend to be much faster" than monolithic or microkernel designs. For example, MIT's Cheetah HTTP static file server (comparable to squid, at the time named Harvest) achieved a 4x performance improvement just by moving to an exokernel and an 8x performance improvement by implementing some of the optimizations I've described in previous comments. This depends on the ability to bypass the privileged servers a microkernel typically uses. Microkernels are nice for security, for a few percent overhead, and that's good, but that's about all they do, so they're not seen as worthwhile for many uses. Exokernels can provide the same level of security while also providing far greater flexibility and performance. This essentially provides, with a performance gain, what current data centers use virtualization for, which is another few-percent-slowdown.